After his win in Indiana, Donald J. Trump has become the GOP’s presumptive nominee for President of the United States. Republicans should be genuinely fearful of what will happen to their party if Mr. Trump manages to win the general election. A Trump presidency would exacerbate the rift in the Republican Party between populists and the establishment, lead to Republican losses in the 2018 midterms and the 2020 presidential election, and turn a generation of young people away from conservative ideals. If you are a Republican who is thinking of holding your nose and voting for Trump, or even just staying home come election day, you should reassess your options. You should do what just a few months ago you would have considered unthinkable: casting your ballot in November for Hillary Rodham Clinton.

A lot has changed since Jeb(!) Bush was the Republican frontrunner. First, a stark divide has emerged in the Republican Party between “establishment” conservatives and those who feel that they have been left behind by their party and the modern economy. These economically marginalized voters disdain free trade because it has disadvantaged them individually, decry immigration because it has resulted in a loss of jobs in their industry, and view the ongoing liberalization of American culture as moral backsliding.

These Republicans are dangerous to the party for two reasons. First, they will contract rather than expand the Republican Party’s appeal. Their harsh, often xenophobic rhetoric on immigration and trade has increasingly alienated the growing contingent of minority voters in the country, the support of whom Republicans need to cultivate if they hope to win future elections. Surveys taken by Univision when Marco Rubio, Ted Cruz, and John Kasich were still in the race showed Trump with the worst unfavorability rating among the Hispanic electorate than any of the other three Republicans. The same surveys showed Trump losing the Hispanic vote to Secretary Clinton by a whopping 57 percent.

It does not have to be this way; after all, Hispanics have the same distribution of ideological beliefs as the general population. Yet Hispanics who identify as conservatives generally still vote for the Democratic Party. These people should fit naturally within the Republican Party, but the divisive rancor which Trump has brought to the surface in the policy matters which matter most to them has discouraged them from embracing the conservative movement. Putting this harsh rhetoric on the back burner by defeating Mr. Trump would enable Republicans to focus on their outreach to these groups and create a powerful new force in the GOP.

Second, the views of “Trump conservatives” are anathema to young people. For better or for worse, young people are future voters, and they view PC culture much more favorably than do those who are older. There is certainly a debate to be had about whether PC culture has gone too far, but Republicans must be aware that having a presidential candidate who makes sexist remarks, advocates discriminating against a class of people based on their religion, and fails to immediately disavow David Duke and the KKK on national television will ultimately alienate young voters. And voters form lifetime party alliances based on the politics during their formative years. If Mr. Trump loses in a landslide, however, this faction of the party will sink back below the Republican Party’s surface, allow the establishment to reassert itself—much as it did when it subsumed the Tea Party in 2014 after the movement suffered losses in the 2012 legislative elections—and build a more inclusive party. But, if Trump wins, this faction will continue pushing its agenda for the foreseeable future. A President Trump would ruin the long-term viability of the Republican Party among young people, turning an entire generation of potential Republicans away from the conservative cause.

A Clinton presidency would give the Republican establishment valuable time to regroup without having to worry about damage control for an erratic President Trump. The Republican National Committee and its backers could instead burnish the national profiles of conservative establishment figures like Paul Ryan and Marco Rubio before the 2018 midterms and 2020 presidential race. In addition to being less divisive and controversial than Trump, Ryan and Rubio are also more popular with the American public. Ryan’s unfavorability rating is 39.0%. Rubio, fresh out of a tough primary season with a barrage of attack ads aimed against him, has an unfavorability rating of 46.0%. Trump’s unfavorability rating is a staggering 61.9%. It is even higher among women, Latinos, black people, and the educated. If this man loses his outsider status and becomes associated with mainstream American conservatism, the Republican Party’s image will be sullied for at least the next decade. Look no further than to what happened to the Democrats after Jimmy Carter, another anti-establishment candidate who inspired an intra-party coalition to block his nomination, was elected President: twelve years of the opposing party controlling the presidency. In contrast, if Republicans abandon Trump in the general election, they could find themselves in a similar situation to the Republican party in 1964, after conservative ideologue Barry Goldwater was defeated in a landslide, with the potential to win the presidency in the next election and the possibility of then holding the presidency for three terms (Republicans weren’t able to keep the White House for three terms because of the Nixon scandal, a one-off event that shouldn’t affect our current calculus).

Indeed, if a Democrat prevails in this presidential election, the Republicans face better odds of winning the White House in 2020 and beyond. Usually, when a president from one party is in power, the opposing party temporarily becomes more popular (The Republicans took advantage of this in the 2010 midterms following President Obama’s election, where Republicans took control of the House and greatly diminished the Democrats’ Senate majority). Because of this upswing in the opposition party’s popularity, the presidency alternates between Republicans and Democrats fairly regularly. The United States has not elected one party to the presidency four times in a row since the time of FDR and Truman. The Democrats’ stranglehold on power in that era was so controversial that a Republican-controlled Congress enacted a constitutional amendment limiting presidents to two terms in office. A one-term Democratic president could boost the Republican Party’s favorability ratings.

A polarizing President Trump, however, would only help the Democrats. As sitting president, Mr. Trump would either secure the 2020 Republican nomination or engage in a bruising primary fight with the eventual nominee (given Trump’s combative personality, he is unlikely to go quietly). Either scenario would severely weaken the GOP candidate and would allow the Democrat—likely to be a gifted young politician like Senator Corey Booker or Senator Martin Heinrich—to win the election in 2020 and enter the 2024 election as an incumbent. To add insult to injury, a Trump administration would not even manage to pass genuinely conservative legislation. From healthcare to Planned Parenthood funding to transgender bathroom access, Mr. Trump has taken liberal stances on a host of policy issues. On the flip-side, a Clinton administration would actually result in more conservative-friendly policies than a Trump administration would, as Secretary Clinton is a foreign-policy hawk, supports free trade, and is unwaveringly pro-Israel.Hillary Clinton is the rational choice for conservative voters this election cycle. Electing a widely loathed demagogue with regressive social policies would irreparably divide the Republican Party. It would also alienate young voters, whose views will be transformed for decades to come by the outcome of this presidential election contest. Among Americans of ages 18-29, Mr. Trump has a net favorability rating—favorables minus unfavorables—of -57. Among Republicans in this age group, this figure is -20, and a quarter of these young Republicans have indicated that they will not vote for Mr. Trump in November if he wins the party’s nomination.

The future of the Republican Party depends on a Clinton victory in 2016. Whatever you think of her policies, President Clinton is unlikely to destroy American conservatism; Donald Trump invariably will.